Doc: comment on "build systems a la carte"; tweaks

You can find the latest version of this manual at

@url{https://common-lisp.net/project/asdf/asdf.html}.

ASDF Copyright @copyright{} 2001-2016 Daniel Barlow and contributors.

ASDF Copyright @copyright{} 2001-2018 Daniel Barlow and contributors.

This manual Copyright @copyright{} 2001-2016 Daniel Barlow and contributors.

This manual Copyright @copyright{} 2001-2018 Daniel Barlow and contributors.

This manual revised @copyright{} 2009-2016 Robert P. Goldman and Francois-Rene Rideau.

This manual revised @copyright{} 2009-2018 Robert P. Goldman and Francois-Rene Rideau.

Permission is hereby granted, free of charge, to any person obtaining

a copy of this software and associated documentation files (the

@cindex :asdf2

@cindex :asdf3

ASDF is Another System Definition Facility:

ASDF, or Another System Definition Facility, is a @emph{build system}:

a tool for specifying how systems of Common Lisp software

are made up of components (sub-systems and files),

and how to operate on these components in the right order

In particular, ASDF should not to be confused with Quicklisp or ASDF-Install,

that attempt to find and download ASDF systems for you.

Despite what the name might suggest,

ASDF-Install was never a part of ASDF, it was a separate piece of software.

ASDF-Install is also unmaintained and obsolete.

ASDF-Install was never a part of ASDF; it was always a separate piece of software.

ASDF-Install has also been unmaintained and obsolete for a very long time.

We recommend you use Quicklisp

(@uref{http://www.quicklisp.org/}) instead,

a Common Lisp package manager which works well and is being actively maintained.

@unnumbered Bibliography

@itemize

@item Andrey Mokhov, Neil Mitchell and Simon Peyton Jones:

``Build Systems à la Carte'', 2018.

  @url{https://www.microsoft.com/en-us/research/uploads/prod/2018/03/build-systems-final.pdf}

  This article provides axes along which to describe build systems in general;

  ASDF, in addition to being in-image (an axis not considered by these authors),

  has the following characteristics:

  ASDF's persistent build information is file modification times

  (the way ASDF is written, it should be easy enough to write an extension that modifies it

  to use a ``cloud cache'' à la Bazel,

  but that would involve using some database, network and cryptographic libraries,

  which cannot reasonably be included in the base ASDF,

  that must remain a minimal bootstrappable system with no external dependencies).

  The object model of ASDF was initially designed for ``static'' dependencies

  with a ``topological'' scheduler, but its @code{defsystem-depends-on} mechanism

  (and more generally, the ability to call ASDF from within an @code{.asd} file)

  allows for multiple @emph{phases} of execution resulting

  in ``dynamic'' dependencies with a ``suspending'' scheduler.

  The rebuilder essentially uses a ``dirty bit'', except that the in-image model

  and the multiple phase support mean that's actually more than a bit:

  instead three bits, the timestamp and a phase depth level.

  The build is guaranteed ``minimal'' in number of steps computed.

  It is local; it assumes but does not enforce determinism;

  it does not assume early cutoff of the build when rebuild dependencies didn't change.

@item Francois-Rene Rideau:

  ``ASDF 3, or Why Lisp is Now an Acceptable Scripting Language'', 2014.

  This article describes the innovations in ASDF 3 and 3.1,